Methods and apparatus for measuring rotating machine clearances

ABSTRACT

A method of monitoring a clearance distance between a rotatable member and a stationary member within a rotary machine is provided. The method includes exciting a probe with a first modulation signal when the rotatable member is rotating at less than or equal to a predetermined rate, exciting the probe with a second modulation signal when the rotatable member is rotating at greater than the predetermined rate, measuring the clearance distance between the rotatable member and the stationary member using the probe excited with at least one of the first modulation signal and the second modulation signal.

BACKGROUND OF THE INVENTION

This invention relates generally to rotary machines, and moreparticularly to a clearance measuring system for determining clearancedistances between rotating rotary machine members.

At least some known rotary machines use capacitance probe-basedclearance measurement systems to monitor rotatable member clearances.Specifically, one such measurement system used in determining turbineblade tip clearance measurement uses a frequency modulated (FM)capacitance probe. Another known system uses DC measurement techniques.FM systems are advantageous in that these systems may be less affectedby gas ionization effects that may be present in gas turbines.Specifically, the capacitance tip clearance system measures thecapacitance between the probe and the blade tip. The measuredcapacitance is then related to tip clearance using a pre-determinedcalibration factor in conjunction with the fundamental relationship forcapacitance, ${C = \frac{E_{r}E_{o}A}{d}},$where E_(r) represents the relative permittivity of the dielectricbetween the electrodes, E_(o) represents the permittivity of free space,A represents the electrode area, and d represents the electrodeseparation. In this case one electrode is the blade tip, the other is aprobe mounted on the engine casing.

However, despite the advantages provided with FM systems, at lowrotational speeds and zero speed, FM capacitance probe clearancemeasurement systems may be ineffective. Specifically, at low speeds, theclearance system accuracy decreases and the clearance output decreasesto substantially zero at zero speed such that measurements areunreliable.

BRIEF DESCRIPTION OF THE INVENTION

In one aspect, a method of determining a clearance distance between arotatable member and a stationary member within a rotary machine isprovided. The method includes exciting a probe with a first modulationsignal when the rotatable member is rotating at less than or equal to apredetermined rate, exciting the probe with a second modulation signalwhen the rotatable member is rotating at greater than the predeterminedrate, measuring the clearance distance between the rotatable member andthe stationary member using the probe excited with at least one of thefirst modulation signal and the second modulation signal.

In another aspect, a clearance measurement system for determining aclearance distance between a rotatable member and a stationary memberwithin a rotary machine is provided. The system includes a probe thatincludes a measurement face that is sensitive to a proximity of therotatable member, a switch, selectable between a first position thatdefines a path from a first pole to a common pole and a second positionthat defines a path from a second pole to the common pole wherein theswitch common pole is electrically coupled to the probe, an amplitudemodulation clearance measurement circuit electrically coupled to thefirst pole of the switch, and a frequency modulation clearancemeasurement circuit electrically coupled to the second pole of theswitch.

In yet another aspect, a rotary machine is provided. The machineincludes a stationary member, a rotatable member, rotatable at leastpartially within the stationary member, a probe mounted in an apertureextending though the stationary member and in communication with therotatable member, an amplitude modulation clearance measurement channelcomprising an amplitude modulation clearance signal amplifier circuitelectrically coupled to an amplitude modulation oscillator, a frequencymodulation clearance measurement channel comprising a frequencymodulation clearance signal amplifier circuit electrically coupled to afrequency modulation oscillator, and a switch that is selectable betweenthe amplitude modulation channel and the frequency modulation channelwherein the switch is electrically coupled to the probe.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective end view of an exemplary rotary machine;

FIG. 2 is schematic diagram of an exemplary clearance measurement systemthat may be used with the rotary machine shown in FIG. 1; and

FIG. 3 is a block diagram of an exemplary method of monitoring aclearance distance between a rotatable member and a stationary memberwithin a rotary machine, such as the machine shown in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a perspective end view of an exemplary rotary machine 100. Inthe exemplary embodiment, machine 100 is a gas turbine such as a Model7FB, commercially available from General Electric, Greenville, S.C.Machine 100 includes a rotatable member 102 and a stationary member 104.Rotatable member 102 may include radially extending members (not shown),such as, but not limited to turbine blades, and is configured to rotateabout a longitudinal axis 106. Stationary member 104 includes at leastone aperture 108 through a sidewall 110. In the exemplary embodiment,aperture 108 includes a mounting adapter 112. In an alternativeembodiment, aperture 108 is threaded to receive a capacitance proximityprobe 114 directly. Probe 114 extends radially inwardly through aperture108 toward rotatable member 102. A sensing end 116 of probe 114 ispositioned a predetermined distance 118 from an outer periphery 120 ofrotatable member 102. Probe 114 is electrically coupled to an electroniccontrol 122 and a computer 124 through one or more instrument cables 126that may be joined together serially using one or more connectors and/ortermination points (not shown). Electronic control 122 is furthercommunicatively coupled to computer 124 to record, display, and processan output of electronic control 122. As used herein, the term computeris not limited to just those integrated circuits referred to in the artas processors, but broadly refers to computers, processors,microcontrollers, microcomputers, programmable logic controllers,application specific integrated circuits, PCs, distributed controlsystems (DCS) and other programmable circuits.

In operation, probe 114 receives an excitation signal from electroniccontrol 122 such that sensing end 116 is capacitively coupled to objectspositioned proximate sensing end 116. In one embodiment, probe 114receives an amplitude modulated excitation signal. In an alternativeembodiment, probe 114 receives a frequency modulated excitation signal.In the exemplary embodiment, probe 114 receives an amplitude modulatedexcitation signal and a frequency modulated excitation signalalternately depending on an operating condition of machine 100 that mayinclude, but is not limited to, a rate of rotation of rotatable member102.

FIG. 2 is schematic diagram of an exemplary clearance measurement system200 that may be used with rotary machine 100 (shown in FIG. 1). System200 includes probe 114 that is electrically coupled to a switch 202through cable 204. Probe 114 is coupled in capacitive communication withrotatable member 102 and mounted to stationary member 104 (shown in FIG.1). Switch 202 includes a common pole 206, a first pole 208 and a secondpole 210, such that, in a first position 212, an electrical path isdefined between common pole 206 and first pole 208, and in a secondposition 214, an electrical path is defined between common pole 206 andsecond pole 210. In the exemplary embodiment, switch 202 is an integralcomponent of electronic control 122, mounted within a common enclosure.In an alternative embodiment, switch 202 is mounted separately fromelectronic control 122 and is electrically coupled through cables 216and 218 to electronic control 122 through panel connectors 220 and 222,respectively. First pole 208 is electrically coupled to a frequencymodulating oscillator 224, which is further coupled to a frequencymodulating (FM) measurement circuit. Second pole 210 is electricallycoupled to an amplitude modulating (AM) oscillator 228, which is furtherelectrically coupled to an amplitude modulating measurement circuit 230.In the exemplary embodiment, oscillator 228 and circuit 230 are mountedwithin electronic control 122 and is used in conjunction with switch 202and oscillator 224 and circuit 226. In an alternative embodiment,electronic control 122 only includes oscillator 224 and circuit 226, andoscillator 228 and circuit 230 are housed in a separate enclosure, andare used instead of oscillator 224 and circuit 226 to excite probe 114and receive signals from probe 114. In the exemplary embodiment, switch202 is automatically selectable based on the operating condition ofmachine 10 (shown in FIG. 1). For example, switch 202 may be configuredto select first position 212 when the rate of rotation of rotatablemember 102 is greater than a predetermined range, such as approximatelyfive-hundred RPM. At a rate of rotation less than five-hundred RPMswitch 202 may select second position 214. Accordingly, switch 202 maybe a relay or other switching device that may be controlled from auser's separate control system and/or other logic or processing device.In an alternative embodiment, switch 202 is configured to be manuallyselectable between fist position 212 and second position 214. Theselection is configured to be made through the use of a user controlsystem (not shown) but, may be configured such that the selection ismade directly manually at switch 202. Although system 200 is illustratedhaving only one probe 114, system 200 may include a plurality of probes114 spaced apart along stationary member 104 such that predeterminedareas of interest are monitored during all operating conditions ofmachine 100. System 200 may also include a respective plurality ofoscillators and measurement circuits coupled to the plurality of probes114.

In operation, oscillator 228 and circuit 230 are electrically coupled toprobe 114 through switch 202. System 200 may be calibrated using acalibration station (not shown). Calibration constants are determinedfrom the calibration and are entered into circuits 226 and 230.Oscillator 228 and circuit 230 may be activated to sense a position ofrotatable member 102. Rotatable member 102 is then rotated manually toposition an area of interest proximate sensing end 116. Distance 118 ismeasured mechanically using a depth micrometer or other measuring means.The mechanically measured distance 118 is compared to distance 118measured by system 200 and further calibration coefficients aredetermined and entered into circuits 226 and 230. During a procedure foraligning the position of rotatable member 102 within stationary member104, the clearance distance between rotatable member 102 and stationarymember 104 is determined using probe 114 that is excited with the firstmodulation signal and the position of rotatable member 102 with respectto stationary member 104 is adjusted using the measured clearancedistance. In the exemplary embodiment, only one probe and associatedelectronic circuits are shown, but it is anticipated that a plurality ofprobes and associated electronics may be used to determine clearances ata plurality of points spaced about rotatable member 102 and stationarymember 104. By comparing clearance distances at a plurality ofmeasurement points, a relative position and orientation of rotatablemember 102 within stationary member 104 may be determined. The positionand orientation of rotatable member 102 within stationary member 104 maybe adjusted to match a predetermined position and orientation tofacilitate aligning rotatable member 102 within stationary member 104.At startup of machine 10, second position 214 is selected to measuremachine cold clearances using AM oscillator 228 and AM circuit 230.System 200 monitors clearances of rotatable member 102 with respect tostationary member 104 as rotatable member increases its rate of rotationusing AM oscillator 228 and measurement electronics 230 until apredetermined range of the rate of rotation is reached, for example,five-hundred RPM. Switch 202 is switched to first position 212 whereinexcitation for probe 114 comes from FM oscillator 224 and the output ofprobe 114 is transmitted to circuit 226. Other operating conditions ofmachine 100 may also be used to determine the position of switch 202.The clearance distance that is measured just prior to switch 202switching from position 214 to position 212 is compared to the clearancedistance that is measured just after switch 202 is switched fromposition 214 to position 212. The clearance distance being measured justprior to switching is being measured by AM oscillator 228 andmeasurement electronics 230. The clearance distance being measured justafter switching is being measured by FM oscillator 224 and measurementelectronics 226. A clearance distance difference greater than apredetermined range may indicate a measurement error. System 200 may usethe difference to modify calibration constants in measurement circuits226 and 230 to correct the clearance measurement and/or may signal analarm indicating a potential error to an operator or a supervisorycontrol system. In the exemplary embodiment, measurement electronicscircuits 226 and 230 each comprise a capacitance displacement transducer(CDT) amplifier that transmits a 0-10 V_(dc) capacitance signal which isfed to a 1/V precision converter to output a linear clearance signalover the range 0.1-10 V_(dc) to computer 124. Computer 124 includes dataacquisition hardware and executes data acquisition software.

FIG. 3 is a block diagram of an exemplary method 300 of monitoring aclearance distance 118 between a rotatable member 102 and a stationarymember 104 within a rotary machine 100, such as machine 100 (shown inFIG. 1). Method 300 includes exciting 302 a probe with a firstmodulation signal when the rotatable member is rotating at less than orequal to a predetermined rate. In the exemplary embodiment, the probe isa capacitance probe that is excited from an AM oscillator with anapproximately sixteen kHz sine-wave signal. Method 300 includes exciting304 the probe with a second modulation signal when the rotatable memberis rotating at greater than the predetermined rate. In the exemplaryembodiment, after the rotatable member reaches a rate of rotation in therange of approximately five-hundred RPM, a switch device, such as arelay, switches excitation of the probe to a FM oscillator andmeasurement circuit. Method 300 includes measuring 306 the clearancedistance between the rotatable member and the stationary member usingthe probe excited with at least one of the first modulation signal andthe second modulation signal. Using an AM excitation source andmeasurement circuit at zero or low rotational speed, and using a FMexcitation source and measurement circuit at relatively higherrotational speeds facilitates improving the accuracy of the measurementsover a wide range of operational conditions. The calibration constantsincluded in software executing on measurement electronics 230 that areused to determine the measured clearance distances may be modified if adifference between the recorded distance value using the firstmodulation signal and the recorded distance value using the secondmodulation signal are outside a predetermined range with respect to eachother. The excitation of the probe may be switched back to the firstmodulation signal if a difference between the recorded distance valueusing the first modulation signal and the recorded distance value usingthe second modulation signal are outside a predetermined range withrespect to each other and modifying the calibration constants does notbring the measurements within the predetermined range. Using the AMexcitation source and measurement circuit prior to startup of themachine allows a convenient method of verifying circuit integrity andoperational readiness.

The above-described rotary machine clearances measurement system iscost-effective and highly reliable for measuring cold clearances betweenmoving parts in the machine prior to startup, for checking the circuitcontinuity and operational readiness of the measurement system, and fordetermining probe position within the machine casing. Specifically, anAM capacitance displacement measuring system is used in conjunction witha FM capacitance displacement measuring system to monitor clearancedistances over the operating speed range of the machine. As a result,the methods and apparatus described herein facilitate more accuratemonitoring of rotating machinery at reduced labor costs in acost-effective and reliable manner.

Exemplary embodiments of rotary machine clearances measurement systemsare described above in detail. The systems are not limited to thespecific embodiments described herein, but rather, components of eachsystem may be utilized independently and separately from othercomponents described herein. Each system component can also be used incombination with other system components.

While the invention has been described in terms of various specificembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theclaims.

1. A method of monitoring a clearance distance between a rotatablemember and a stationary member within a rotary machine, said methodcomprising: exciting a probe with a first modulation signal when therotatable member is rotating at less than or equal to a predeterminedrate of rotation; exciting the probe with a second modulation signalwhen the rotatable member is rotating at greater than the predeterminedrate of rotation, said second modulation signal is different than thefirst modulation signal; and switching the excitation of the probe fromthe first modulation signal to the second modulation signalautomatically at the predetermined rate of rotation; measuring theclearance distance between the rotatable member and the stationarymember using the probe excited with at least one of the first modulationsignal and the second modulation signal.
 2. A method in accordance withclaim 1 wherein exciting a probe with a first modulation signalcomprises exciting a probe with an amplitude modulation signal.
 3. Amethod in accordance with claim 1 wherein exciting a probe with a secondmodulation signal comprises exciting a probe with a frequency modulationsignal.
 4. A method in accordance with claim 1 further comprising:recording a clearance distance value using the first modulation signaljust prior to switching from the first modulation signal to the secondmodulation signal; switching the excitation of the probe from the firstmodulation signal to the second modulation signal automatically at thepredetermined rate of rotation; recording the clearance distance valueusing the second modulation signal just after switching from the firstmodulation signal to the second modulation signal; and modifying atleast one calibration constant of a software calibration equation if adifference between the recorded distance value using the firstmodulation signal and the recorded distance value using the secondmodulation signal are outside a predetermined range with respect to eachother.
 5. A method in accordance with claim 4 further comprisinggenerating an alarm signal if a difference between the recorded distancevalue using the first modulation signal and the recorded distance valueusing the second modulation signal are outside a predetermined rangewith respect to each other.
 6. A method in accordance with claim 4further comprising switching the excitation of the probe from the secondmodulation signal to the first modulation signal automatically if adifference between the recorded distance value using the firstmodulation signal and the recorded distance value using the secondmodulation signal are outside a predetermined range with respect to eachother.
 7. A method in accordance with claim 1 further comprisingswitching excitation of the probe from the first modulation signal tothe second modulation signal manually.
 8. A method in accordance withclaim 1 further comprising measuring the clearance distance between thestationary member and the rotatable member substantially continuouslybetween a zero rotational speed and a predetermined rotational overspeedlimit.
 9. A method in accordance with claim 1 further comprising:measuring the clearance distance between the rotatable member and thestationary member using the probe excited with the first modulationsignal; and adjusting the position and orientation of the rotatablemember with respect to the stationary member using the measuredclearance distance to align the rotatable member within the stationarymember.
 10. A method in accordance with claim 1 wherein the probe isselectively, electrically coupled to at least one of a first clearancesignal amplifier circuit through the first modulation signal oscillator,and a second clearance signal amplifier circuit through the secondmodulation signal oscillator, and wherein measuring the clearancedistance between the rotatable member and the stationary membercomprises: receiving a probe signal by at least one of the firstclearance signal amplifier circuit and the second clearance signalamplifier circuit; correlating the received signal to a clearancedistance between the rotatable member and the stationary member;measuring the clearance between the rotatable member and the stationarymember; and calibrating the first and second clearance signal amplifiercircuit based on the measured clearance.
 11. A method in accordance withclaim 10 wherein measuring the clearance distance between the rotatablemember and the stationary member comprises measuring the cold clearancedistance between the rotatable member and the stationary member.
 12. Amethod in accordance with claim 11 wherein the probe is mounted in anaperture that extends through the stationary member, and whereinmeasuring the cold clearance distance comprises measuring the coldclearance distance using a mechanical measurement device that at leastpartially extends through the aperture to contact the rotatable member.13. A clearance measurement system for monitoring a clearance distancebetween a rotatable member and a stationary member within a rotarymachine, said system comprising: a probe comprising a measurement facethat is sensitive to a proximity of said rotatable member; a switch,selectable between a first position that defines a path from a firstpole to a common pole and a second position that defines a path from asecond pole to the common pole, said switch common pole electricallycoupled to said probe; an amplitude modulation clearance measurementcircuit electrically coupled to the first pole of said switch; and afrequency modulation clearance measurement circuit electrically coupledto the second pole of said switch.
 14. A clearance measurement system inaccordance with claim 13 wherein said rotatable member is a turbinerotor assembly.
 15. A clearance measurement system in accordance withclaim 13 wherein said probe is a capacitance probe.
 16. A clearancemeasurement system in accordance with claim 13 further comprising aplurality of probes, each said probe associated with a switch, anamplitude modulation clearance measurement circuit, and a frequencymodulation clearance measurement circuit.
 17. A clearance measurementsystem in accordance with claim 13 wherein said switch is selectable atleast one of manual input from a user and automatically based on apredetermined operating condition of said rotary machine.
 18. Aclearance measurement system in accordance with claim 17 wherein saidswitch is selectable based on a predetermined rate of rotation of saidrotatable member.
 19. A clearance measurement system in accordance withclaim 18 wherein said switch selects the first position when therotatable member rate of rotation is less than the predetermined rate ofrotation.
 20. A clearance measurement system in accordance with claim 18wherein said switch selects the second position when the rotatablemember rate of rotation is greater than or equal to the predeterminedrate of rotation.
 21. A rotary machine comprising: a stationary member;a rotatable member, rotatable at least partially within said stationarymember; a probe mounted in an aperture extending though said stationarymember and in communication with said rotatable member; an amplitudemodulation clearance measurement channel comprising an amplitudemodulation clearance signal amplifier circuit electrically coupled to anamplitude modulation oscillator; a frequency modulation clearancemeasurement channel comprising an frequency modulation clearance signalamplifier circuit electrically coupled to a frequency modulationoscillator; and a switch selectable between said amplitude modulationchannel and said frequency modulation channel electrically coupled tosaid probe.
 22. A rotary machine in accordance with claim 21 whereinsaid probe comprises a capacitance probe configured to receive anexcitation signal and to generate an output signal related to aproximity of said inner member to a measurement face of said probe. 23.A rotary machine in accordance with claim 22 wherein said probe isconfigured to receive a frequency modulated excitation signal.
 24. Arotary machine in accordance with claim 22 wherein said probe isconfigured to receive an amplitude modulated excitation signal.
 25. Arotary machine in accordance with claim 21 wherein said switch isselectable at least one of manually and automatically based on apredetermined rate of rotation of said rotatable member.